kontrolltroll: Let him talk to you, fight him till you get the dialog where he tells you he is immortal, then flee out of the dungeon and talk to the herbalist woman, buy some holy water and fight him again.
TheDarkSlayer: Unfortunately, i never spoke the first time with the knight, because i attacked him immediatly with magic and my followers blocked is way, so i was only able to get the second dialogue, when he says that he will live thanks to his victims, but this dialogue does not update my journal and i have nothing to say to the herbalist.
It is the correct dialogue when he says that he will live thanks to his victims! You're journal will not be updated after he said that and you left the dungeon. But there must be an option to talk to Enrie about his immortality, just try to click trough all dialog option with her. After that you should buy some holy water and fight again against him and he should fall. If you don't get any option to talk to Enrie about it, try to buy holy water without that and fight again against him.
LordCrash: Could anybody please tell me where to find Devil's Cavern? I've marched through every map around Glatzburg and I couldn't find it?!
michael.ch: some npc tells where devil's caverns are.
Thanks, I've found out on my own a few minutes ago. I just had to talk to (or to torture) the right person. ;)